Prepare for the Possibility of Postbloom Fruit Drop 

Josh McGill Diseases, Tip of the Week

By Megan M. Dewdney

Postbloom fruit drop (PFD) is a sporadic flower disease. It is primarily caused by the fungus Colletotrichum acutatum. While infections only occur on flowers, the fungus is present on trees throughout the year. It survives by producing resting structures on leaves and stems. How the inoculum survives multiple years between infections is still unclear. The fungus is stimulated out of its resting state to produce spores by substances from the early flowers. If conditions are favorable, these early spores infect the initial flowers and form many more spores that can cause greater infection.

However, flower infections only occur with specific weather conditions. Moderately warm temperatures, between 72 to 79 degrees, and long leaf wetness periods of more than 16 hours (usually associated with rainfall) are perfect for the fungus. If the temperatures are cooler or warmer, the wetting period needs to be longer for a successful infection event.

If infection occurs on successive waves of early flowers before the major bloom period, the number of spores greatly increases the likelihood of a PFD outbreak with the right weather during the major bloom. That is why it is important to scout the early bloom for PFD. Knowing whether you have active inoculum in your groves will allow you to plan for potential infection events if rain occurs during bloom.

If symptoms are present, use the Citrus Advisory System – AgroClimate to identify periods of increased PFD risk in your area when the main bloom comes. If symptoms are not present, decide whether PFD is a worry for your location based on your recent PFD history.

Postbloom fruit drop
Figure 1. Orange-colored lesions on some of the petals are typical postbloom fruit drop symptoms on sweet orange flowers.

Outbreaks of PFD have been localized in Florida since 2016, so typical PFD symptoms might be fading from memory. Popcorn and opened flowers are the most susceptible to the disease, so concentrate scouting for symptoms on these stages. PFD lesions are peach to pinkish-brown on flower petals (Figure 1). Whole flower clusters are affected when PFD conditions are perfect. With optimal temperatures, flowers and symptoms may develop so quickly they are missed when scouting because the petals fall.

In looking at the seasonal forecast for February to April, Florida temperatures are likely to be above average, but lower than average rainfall is expected. However, it only takes rain at the wrong time if you have inoculum present to get an outbreak. That is why it is crucial to know how to identify PFD symptoms on fruitlets and calyces once the flowers fall. Fruitlets from PFD-affected flowers turn chlorotic and fall off trees, leaving behind the persistent calyces, also known as PFD buttons (Figure 2).

Postbloom fruit drop
Figure 2. Fruitlets from flowers affected by postbloom fruit drop turn chlorotic and fall off trees, leaving behind the persistent calyces, also known as buttons.

Bloom on trees is starting to be seen, but I have not observed buttons or heard reports of them yet. PFD buttons can be differentiated from calyces caused by natural fruit fall when trying to remove them from trees. While calyces from natural fruit fall are easily removed, PFD buttons are not easily taken off trees. Identifying PFD on early blooms is crucial for successful PFD management programs when the main bloom comes.

Megan M. Dewdney is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.

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